Steam reforming is a widely used method for production of hydrogen. Steam reforming can be utilized, for example, in applications such as synthetic gas (syngas) production and cleanup of the syngas. Syngas can be produced, for example, from a solid hydrocarbon material such as coal or biomass via gasification processes in which steam is combined with oxygen to convert the solid hydrocarbon material into its gaseous derivative components, including hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), water (H2O), methane (CH4) as well as tar and other light hydrocarbons. A significant and expensive challenge during coal or biomass gasification is cleanup of the raw syngas, in which certain gaseous components (e.g., methane and tar) are reformed and other contaminants (e.g., ammonia, hydrochloric acid, sulfur, mercury and other heavy metals) are removed. In addition, depending upon the application of the syngas, the syngas may further need to be conditioned to adjust the hydrogen-to-carbon monoxide ratio in order to meet downstream process requirements. Steam reforming can be utilized in the coal gasification process to convert light hydrocarbon species such as methane to hydrogen and one or more other components (e.g., carbon monoxide).
In a steam reforming process, methane and/or other light hydrocarbons are contacted with an oxidant such as steam (H2O) at an elevated temperature to produce a mixture of carbon monoxide and hydrogen. The formed carbon monoxide can then be reacted with steam at relatively lower temperature to produce carbon dioxide and hydrogen. A methane reforming reaction is highly endothermic and thus requires high energy input to break the C—H bond in methane. The required heat energy input can be provided by indirect heat exchange using an external heat source (e.g., a burner or firebox).
Reforming can also be performed in two direct heating configurations, namely, partial oxidation and auto thermal reforming. In partial oxidation, instead of using steam, methane is oxidized by reacting with oxygen to form carbon monoxide and hydrogen. This reaction is exothermic. In an auto thermal reforming reaction, both steam and oxygen are used as oxidizing agents (with no external heat being required). The endothermic heat for steam reforming is provided by oxidation and partial oxidation reactions.
Nickel based catalysts that are typically used in reforming operations include nickel metal dispersed on refractory oxides such as α-alumina and MgAl2O4. However, the use of such catalysts face several challenges, such as activity loss due to coking, thermal sintering, sulfur poisoning and metal sintering due to sulfur attack. Sulfur contaminants present in hydrocarbon streams are present as or are converted to hydrogen sulfide (H2S) during high temperature reforming. Sulfur can also be present in other forms, such as mercaptans and thiophenes, and all of these forms of sulfur can poison the nickel based catalyst. In particular, conventional nickel oxide catalysts undergo rapid deactivation in the presence of sulfur, resulting in unacceptably low levels of methane and/or hydrocarbon conversion during the reforming process.
In an effort to achieve greater sulfur tolerance for the catalyst during reforming applications, two known approaches have been considered: use of noble metals in the catalyst which show low equilibrium concentration with sulfur at varied temperature ranges; and use of nickel based catalysts with dopants to increase sulfur tolerance (this approach has largely met with failure). Platinum and other noble metal catalysts show high activity in reforming processes. However, the rate of deactivation caused by sulfur for noble metal based catalysts is still unacceptably high. In addition, noble metal catalysts tend to be too expensive for use in large scale reforming applications. The nickel based catalysts with dopants are also deactivated in the presence of high sulfur concentrations. Thus, no known catalysts in use and in literature studies appear operable for sustained time periods (e.g., 8 hours or more) in the presence of H2S concentrations greater than 100 ppm.
It would be desirable to provide nickel based catalysts for a steam reforming process that facilitate high temperature conversion of methane, tars and other light hydrocarbon species in the presence of contaminants such as H2S and over extended time periods, where H2S is present at concentrations greater than 100 ppm.